Digitally controlled oscillators generate a pulsed output signal having a selected frequency. The output signal is generated in response to a digital input, usually an N-bit code word. In some applications, an oscillator having a high frequency resolution is desirable. Frequency resolution refers to the minimum step size in a range of potential frequencies output by the oscillator. Smaller step sizes allow for a greater number of frequencies to be output within the same frequency range. Therefore, reducing step size (i.e., increasing resolution) means that a target output frequency can be specified with greater precision.
A conventional approach is to use a Direct Digital Synthesizer (DDS) solution in which a high frequency reference clock is used to obtain as output a lower frequency with a large resolution. The DDS solution has a disadvantage in that the frequency is only exact in average value, i.e., the frequency is prone to instantaneous variations and the output frequency is stable only when viewed as an average signal over time. When viewed at a low time scale, the generated output period has one reference period uncertainty for a pure digital DDS solution.
Another approach is using an oscillator whose frequency can be instantaneously changed by a digital word by means of a code-to-frequency conversion sub-circuit (such as a current or a voltage digital-to-analog converter (DAC) inside an oscillator core circuit). In this approach, increasing resolution means to increase the number of bits in the input signal to the oscillator and therefore in the code-to-frequency sub-circuit. The addition of these additional bits allows for a larger set of inputs (and therefore a larger set frequencies that can be expressed along the output range). However, increasing the number of input bits requires large amounts of physical space in an oscillator circuit due to the increase of the code-to-frequency sub-circuit (e.g., DAC). In many cases, each extra bit is accompanied by a two-fold increase in area. Therefore, the required area often exceeds space constraints.